The present invention relates to an electromagnetic vibration generator including an electromagnet made of a U-shaped magnetic core and an excitation winding encased in cast resin in a housing and an armature which faces the pole faces of the magnetic core at a distance constituted by an air gap and which vibrates with a predetermined amplitude.
An electromagnetic vibration generator of the above-outlined type is disclosed in German Patent No. 1,106,103. Such electromagnetic vibration generators are used in various configurations as magnetic vibrators for driving vibratory conveyor troughs for bulk materials and small pieces as well as for driving screens, separators or dosaging devices.
Electromagnetic vibration generators constitute a vibratory system composed of two masses in which the magnet system and the armature vibrate in opposite directions relative to one another. The magnet system and the armature are generally coupled together by means of springs. While the vibration generator is generally excited at the single or double mains frequency, deviations from these driving frequencies are feasible for special applications.
While in earlier prior art systems, the electromagnet has been fastened to a supporting member, for example a housing, with the aid of fastening straps or bolts, in more current designs the electromagnet, that is, the U-shaped magnetic core and its excitation winding, has been encased in resin cast in a housing. Such a solution not only increases the electrical insulation and reduces the weight of the magnetic components, but also considerably saves manufacturing costs.
The air gap defined between the armature and the pole faces in the de-energized state must be of such a dimension that during operation (that is, the energized state) the armature vibrates within the air gap at an amplitude which is less than the width of the air gap. Only this measure can prevent the electromagnet and the armature from striking one another which would destroy the drive system.
While thus, on the one hand, the air gap must be sufficiently wide so that a collision between the electromagnet and the armature is reliably avoided, it is, on the other hand, a desideratum to maintain the air gap small to thus maintain the required inductive magnetization current as small as possible which is desirable with respect to current loads and the design of the mains device. The air gap at rest should thus expediently be dimensioned such that both requirements, that is, the avoidance of striking contact between the vibrating components and the lowest possible power consumption from the mains device are met. Since, moreover, the natural frequency and thus the vibration amplitude also change as a function of the weight of the load--in a conveyor, for example, the material being conveyed--electromagnetic vibratory drives are operated with a controlled supply voltage so as to be able to compensate at least for changes in the weight of the load.
It is an additional problem encountered in prior art constructions that the components and materials of the magnetic system change in volume and geometry in dependence on the temperature. For example, the cast resins employed have a higher coefficient of expansion than steel or even the frequently employed gray cast iron housing. Further, it has to be taken into consideration that the housing is fastened by brackets to a supporting element, and thus the temperature-caused changes in the brackets and in the supporting structure may also result in a change of the air gap dimension. Thus, the thermal expansion that occurs at the operating temperature of the device must also be considered when dimensioning the air gap. This, however, increases the inductive load current in the cold state.